1. Field of the Invention
The present invention relates to a charging apparatus for use in a cordless telephone system.
2. Description of the Prior Art
Heretofore, a cordless sub-telephone of a cordless telephone system is operated by a chargeable battery, which is charged by a charging apparatus or charger (provided in a base station telephone, for example).
Referring now to the drawings, a conventional charging apparatus is described with reference to a cordless telephone system as an example. FIG. 4 illustrates a charging unit of the base station telephone. In FIG. 4, numeral 1 denotes an AC adapter including an AC-to-DC converting circuit and numeral 2 denotes a voltage stabilizing circuit or a constant-voltage circuit connected to the AC adapter to which a control terminal 3 is connected. The constant-voltage circuit 2 can switch a charging current to 0.5, 0.1 and 0.03 of a battery capacity. These currents are hereinafter named a rapidly charging current, a normally charging current and a trickle charging current, respectively. Further, numeral 4 denotes charging terminals connected to an output of the constant-voltage circuit 2.
FIG. 5 illustrates a charging unit of a cordless sub-telephone of the cordless telephone system. In FIG. 5, numeral 5 denotes a secondary battery, 6 charging terminals, 7 a reverse current preventing diode, 8 an input circuit for detecting a voltage of the battery, 9 a voltage reduction detecting circuit, and 10 a microcomputer for producing a control signal to a control terminal 13 in response to an output of the voltage reduction detecting circuit 9. When the microcomputer judges a reduction in the voltage of the secondary battery in accordance with the output of the voltage reduction detecting circuit 9, the microcomputer 10 drives a drive transistor 11 to turn on and off a light emitting diode 12.
Operation of the cordless telephone system configured above is now described.
In the cordless sub-telephone, a voltage of the secondary battery 5 is divided by the input circuit 8 and is supplied to the voltage reduction detecting circuit 9. At this time, when the microcomputer 10 judges from the output of the voltage reduction detecting circuit 9 that a terminal voltage of the secondary battery 5 is lower than a reference voltage, the microcomputer drives the drive transistor 11 to turn on and off the emitting diode 12. At this time, when the cordless sub-telephone is placed on the base station telephone, the control terminal 3 and the charging terminals 4 of the base station telephone come into contact with the control terminal 13 and the charging terminals 6 of the cordless sub-telephone, respectively. Thus, the microcomputer 10 produces a signal indicative of a rapid charge to the connection terminal 13.
The constant-voltage circuit 2 which has received the signal supplies the rapidly charging current to the secondary battery for a fixed time and then supplies the normally charging current to the secondary battery 5. Thereafter, when the terminal voltage of the secondary battery 5 reaches a predetermined value or more, the constant-voltage circuit 2 supplies the trickle charging current to the secondary battery 5 to continues to supplement the naturally discharged component of the secondary battery 5.
When the cordless sub-telephone is placed on the base station telephone when the voltage of the secondary battery 5 is lower than the reference voltage, the micro-computer 10 produces a signal to the connection terminal 13 to flow the normally charging current. The constant-voltage circuit 2 supplies the normally charging current to the secondary battery 5 in accordance with the amount consumed and then supplies the trickle charging current to the battery.
Normally, the secondary battery 5 uses a nickel-cadmium battery, which has a small internal resistance and small variation in voltage even if the residual battery capacity is very small. Further, if a plurality of battery cells are used, voltage values of the battery cells which have been completely charged are influenced by scattered characteristics of each of the battery cells. Accordingly, a battery pack composed of a plurality of battery cells connected in series to each other has a different voltage value for each battery pack used when the battery pack has been charged completely and it is difficult to judge whether the battery pack has been charged completely or not. Thus, when the cordless sub-telephone is returned to the base station telephone to charge the battery after the cordless sub-station has been utilized for a short time, it is judged that a voltage of the battery reaches a value obtained when the battery has been charged completely although the capacity of the secondary battery 5 is reduced slightly, so that the secondary battery 5 is supplied with the trickle charging current from the beginning and the secondary battery 5 is not completely recharged to the fully charged state.